Evaluating Nutrient Fate and Redox Controls in Groundwater in Riparian Areas

Introduction

Elevated nutrient concentrations are a common problem in many watersheds from wastewater, agricultural, and landfill sources. Because nitrogen compounds are mobile in water compared to phosphorus, transport of nitrogen in Groundwater is of particular concern, both as a Groundwater contamination problem and as a source of surface-water contamination by discharge to riparian and estuarine areas. A nitrogen concentration of 1 mg/L or greater is used by the Chesapeake Bay Program for indicating anthropogenic impact.

Background: Nitrogen Fate

Physical, chemical, and biological processes can transform nutrients during transport in Groundwater before discharge to surface water. For example, nitrogen compounds in septic waste typically are converted to nitrate (NO3-) in aerobic drainfields and soils. If nitrate encounters reducing conditions during Groundwater transport, the nitrate concentrations could be decreased by reduction to nitrogen gas by denitrifying bacteria. Wetlands and organic-rich sediment in riparian zones can provide anoxic areas for rapid denitrification of nitrate from Groundwater sources, potentially controlling release of nutrients to surface water. In contrast, ammonium (NH4+), a common form of nitrogen in landfill plumes, can be transformed to nitrate in oxic Groundwater, including near surface wetland and bottom sediments. Identifying geochemical and hydrologic factors that could control such transformation processes is important to evaluate the impact of Groundwater sources to riparian areas.

Groundwater with a source of organic carbon or other electron donors shows a sequence of distinct redox zones along flowpaths. Similar patterns can be seen along upward flowpaths in discharge areas in organic-rich wetland and bottom sediments.

Background: Hydrology in Areas of Groundwater/Surface-Water (GW/SW) Interaction

Tracking Groundwater sources in discharge areas and defining Groundwater/surface-water interactions can be complicated by mixing in the discharge area. Discharge areas may represent Groundwater that has been transported along short, shallow flowpaths and deeper, long flowpaths, in addition to mixing with surface water. Geochemical and microbial reactions can alter the source signature of the water before or within the discharge area.

Conceptual model of GW/SW transition zone. (A) Chemical fluxes in the GW/SW transition zone (Zone 2) have both surface-water (Zone 3) and Groundwater (Zone 1) sources. Fluctuations in hydraulic head can occur, changing the direction of water flow. (B) The sediment within the GW/SW transition zone includes bottom sediments and aquifer solids, and (C) plant and animal biomass.

Field Site Descriptions

Norman Landfill Research Site, Oklahoma. USGS Toxic Substances Hydrology Program

A contaminant plume emanating from a closed municipal landfill in Norman, Oklahoma flows in an alluvial aquifer underneath a riparian wetland (slough). Ammonium in the leachate plume is attenuated compared to other constituents in the plume and appeared to stop at the edge of the slough. We began an investigation to determine the interaction of the plume with the slough, and its role in the fate of ammonium and other leachate constituents.

Port Tobacco River Watershed, Charles County, Maryland.

Septic system failure in seven major residential areas has been identified as one of the potential anthropogenic sources of bacteria and nutrients in surface water in the Port Tobacco River watershed.

The Port Tobacco Riviera community was selected as a representative area for a Groundwater study by the USGS in cooperation with the Port Tobacco River Conservancy, Inc.

Evaluation Methods

Seepage meters and specific conductivity measurements in the slough sediments were used at the Norman Landfill site to evaluate discharge of the plume to surface water. Groundwater flux was correlated to rainfall.

Aerial high-resolution Thermal Infrared Imaging (TIR) was used in the Port Tobacco River watershed to identify possible Groundwater discharge areas, including diffuse discharge and areas of preferential flow (seeps).

Aerial high-resolution Thermal Infrared Imaging (TIR) was used in the Port Tobacco River watershed to identify possible Groundwater discharge areas, including diffuse discharge and areas of preferential flow (seeps).

Groundwater was collected with porous diffusion samplers, or peepers, and multilevel samplers. In situ redox measurements were made at Norman Landfill site using a microelectrode system.

Results

Norman Landfill Research Site, Oklahoma.

Peeper Samples of Slough Porewater

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Seasonal variability in interactions of the leachate plume with the slough was evident in peeper samples. During dry conditions in August 2002, vertically upward discharge was focused at the center of the slough (site 102) from the fringe of the ammonium plume in the underlying aquifer, resulting in low flux of ammonium to the slough. During wet conditions in May 2003, leachate-contaminated groundwater discharged vertically upward near the upgradient slough bank (site 101), and high concentrations of ammonium and other leachate constituents were then transported laterally through the slough porewater to the downgradient bank (site 103). Leachate-associated constituents more than doubled in concentrations in the slough porewater during wet conditions.

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(Note the change in scale for figures b and c. )

Nitrogen isotopes were used to help elucidate the mechanism of ammonium attenuation in the aquifer. Laterally along the slough transect, values of d15N decreased from well SI100 and SI101 to downgradient wells SI102 and SI103 as ammonium concentrations decreased, which can be attributed to cation exchange (or sorption). This trend was also seen at depth in the aquifer at the multilevel samplers.

High dissolved ferrous iron and methane in peeper samples of slough porewater compared to the underlying aquifer indicated anaerobic conditions were present in the naturally organic-rich wetland sediments. Precipitation of ferrous sulfides may decrease iron concentrations when there is an influx of sulfate in dry conditions and during storm events.

Microelectrode measurements showed aqueous FeS was a dominant form of iron in the slough porewater.

Multilevel samplers in the upper 1.0 m of slough sediment and in situ microelectrode measurements also showed high variability of redox constituents during storm events.

Port Tobacco River Watershed, Maryland

Intermittent streams that flow in ravines to the Port Tobacco River had moderate nitrogen in the form of nitrate. Concentrations decreased downstream, however, indicating that surface water was not a major transport mechanism to the river.

Groundwater in sands and gravels in the uplands had moderate nitrogen concentrations in the form of nitrate, and a specific conductance less than 500 µS/cm), as observed in the intermittent streams. In contrast, ammonium and specific conductance greater than about 1,000 µS/cm was found in the shallow Nanjemoy Formation adjacent to the river and in seeps in the riverbed.

Shallow Groundwater (~ 20 ft bls) in the Nanjemoy Formation adjacent to the river was similar to seep porewater (~ 2 ft bls) in riverbed, showing possible river water infiltration.

A conceptual model of nitrogen transport and fate along the Port Tobacco River was developed. Nitrate in aerobic upland deposits may flow to the Nanjemoy close to the river and be reduced to ammonium. Alternatively, river porewater or transport from deep flowpaths in the Nanjemoy may be a source of ammonium.